This invention relates to a process for improving the bulk oxidation stability and storage stability of lube oil base stocks derived from hydrocracked bright stock.
The term "oxidation stability" refers to the resistance of the oil to oxygen addition, in other words, how rapidly is oxygen picked up by and added to molecular species within the oil. Oxidation stability is indicated by the oxidator BN measured in hours. Oxidator BN is thoroughly described in U.S. Pat. No. 3,852,207 granted Dec. 3, 1974 to B. E. Stangeland et al at column 6, lines 15-30. Basically, the test measures the time required for 100 grams of oil to absorb one liter of oxygen. The term "storage stability" refers to the resistance of the oil to floc formation in the presence of oxygen.
The process comprises three steps. In the first step a hydrocracked bright stock is hydrodenitrified to reduce its heteroatom, particularly nitrogen, content using, for example, a sulfided nickel-tin catalyst having a siliceous matrix or a nickel-molybdenum hydrotreating catalyst having an alumina matrix. In the second step, the hydrocracked bright stock, having a reduced nitrogen content, is hydrofinished using, for example, an unsulfided nickel-tin or palladium hydrotreating catalyst having a siliceous matrix. In the third step the hydrotreated-hydrofinished bright stock is subjected to nonhydrogenative acid stabilization.
The first two steps are carried out at an unusually low liquid hourly space velocity (LHSV), about 0.25 Hr.sup.-1. In the first step, a low LHSV permits the desired hydrodenitrification reaction to proceed at relatively low temperatures, about 700.degree. F. Under these conditions hydrocracking is minimized. In the second step a low LHSV permits thorough saturation of aromatics which are floc-forming species. Thus, in general, the first step removes nitrogen and sulfur, known catalyst poisons, and improves oxidation stability; the second step saturates aromatic floc precursors, and improves storage stability; and the third step alkylates any remaining floc precursors increasing their solubility and resistance to floc formation, further improving storage stability. Accordingly, it has been found that the stability of the resultant lube oil base stock is significantly improved. Moreover, since the second and third steps are both directed to improving storage stability, a beneficial effect of the third step is that of reducing the severity of the hydrofinishing conditions required by the second step. This permits either a longer catalyst life or smaller reactors.
Lubricant refining is based upon the fact that crude oils, as shown by experience or by assay, contain a quantity of lubricant base stocks having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate a lubricant base stock consists of a set of unit operations to remove or convert the unwanted components. The most common of these unit operations include, for instance, distillation, hydrocracking, dewaxing, and hydrogenation.
The lubricant base stock, isolated by these refining operations, may be used as such as a lubricant, or it may be blended with another lubricant base stock having somewhat different properties. Or, the base stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and viscosity index improvers. As used herein, the term "stock", regardless whether or not the term is further qualified, refers to a hydrocarbon oil without additives. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "base stock" will refer to an oil refined to a point suitable for some particular end use, such as for preparing automotive oils.
In general, refineries do not manufacture a single lube base stock but rather process at least one distillate fraction and one residuum fraction to produce several lube base stocks. Typically, three distillate fractions differing in boiling range and the residuum of a vacuum distillation operation are refined. These four fractions have acquired various names in the refining art, the most volatile distillate fraction often being referred to as the "light neutral" oil. The other distillates are called "medium neutral" and "heavy neutral" oils. The residuum fraction, is commonly referred to as "bright stock". Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks, which slate may include a bright stock.
Processes have been proposed to produce lubricating oil base stocks by refining bright stocks. Most such refining processes require hydrocracking the bright stock to produce a hydrocrackate which is in turn dewaxed to produce a dewaxed bright stock. The problem is that lubricating oil base stocks derived from hydrocracked stocks are unstable in the presence of oxygen and light.
Various stabilizing steps have been proposed. U.S. Pat. Nos. 3,189,540, 3,256,175 granted June 15, 1965 and June 14, 1966, respectively, to Kozlowski et al, describe a typical stabilization. The proposed stabilization uses a series of process steps employing a severe catalytic hydrogenation step to convert the remaining aromatic constituents into desirable lubricating oil con- stituents.
The goal of hydrogenation is to hydrogenate the unstable species, which are thought to be partially saturated polycyclic compounds. Unfortunately, severe hydrogenation of hydrocracked bright stocks not only hydrogenates the undesirable polycyclic constituents, but also further hydrocracks desirable constituents resulting in the loss of valuable lubricant base stock. Thus, recent processing schemes have suggested several alternatives to severe hydrogenation.
Refiners often now use mild hydrogenation (sometimes referred to as hydrofinishing) to produce more stable lubricating oils. Obviously, mild hydrogenation requires a compromise between the desired stabilization and the undesired hydrocracking. Consequently, thorough stabilization is often not accomplished.
As an alternative to hydrofinishing, stabilizing agents, such as olefins, alcohols, esters, or alkylhalides can be added to the hydrocracked base stock in the presence of acidic catalysts having controlled alkylation activity. The resulting alkylation stabilizes the aromatic floc formers. Some of the earlier efforts concentrated on the addition of stabilizing agents to a dewaxed hydrocrackate while in the presence of a heterogeneous acidic catalyst. Several issued patents relate to stabilizing hydrocracked lubricant base stocks by adding stabilizing agents such as olefins, alcohols, esters or alkylhalides to the lube stock while in the presence of a heterogeneous acidic catalyst such as acid resins, clays, and aluminosilicates having controlled alkylation activity. For instance, U.S. Pat. No. 3,928,171, Yan et et al, Dec. 23, 1975 and U.S. Pat. No. 4,181,597, Yan et et al, Jan. 1, 1980 disclose processes for stabilizing hydrocracked lube oils which have been dewaxed, preferably solvent dewaxed, by contacting them with stabilizing agents such as C.sub.6 to C.sub.10 olefins. The stabilizing catalyst is a heterogeneous acidic catalyst; the olefins which are not consumed in the reaction are distilled off from the product and combined with other olefins to be added into the stabilizing reaction zone feed. As discussed in U.S. Pat. No. 4,181,597 the mechanism responsible for the benefits obtained when using a stabilizing agent was not entirely understood. However, since the stabilizing agent is consumed during the stabilization reaction, it is likely that a reaction occurs between one or more components of the dewaxed lube oil stock and the stabilizing agent. In particular, conditions during the stabilization process are conducive to alkylation. While these and other processing schemes have achieved some success, in the case of highly aromatic stocks, such as bright stock, none of the previously known schemes is entirely satisfactory.
Thus, in general, at the time of the present invention, the literature relating to lube oil stabilization taught the use of severe hydrogenation or, alternatively, mild hydrofinishing and/or alkylation to stabilize a hydrocracked bright stock. However, in spite of the large amount of research into developing lubricant base stocks and stabilizing them, there continues to be intensive research into developing a more efficient and more convenient method for achieving those goals, especially for lubricant base stocks derived from hydrocracked bright stocks. The object of the present invention is to provide such a process.
It has now been discovered that a three-step hydrogenation/alkylation process comprising a first step to reduce the nitrogen and sulfur content, a second step to hydrogenate unstable polycyclics, and a third step to alkylate floc precursors increasing their solubility and resistance to floc formation will produce a more stable lubricating oil base stock from hydrocracked bright stock. Thus, rather than employing a single severe hydrogenation step, the present invention employs a relatively milder three-step hydrofinishing and nonhydrogenative stabilization for hydrocracked bright stocks.